Two-dimensional angular correlation: comparison between theory and experiment

A brief introduction to the comparison between theory and experiment for the two-dimensional angular correlation of positron annihilation radiation (2D ACPAR) is given. The methods discussed are: (i) Comparison of the experimental 2D ACPAR curves with those obtained from band structure calculations, (ii) Reconstruction of two-photon momentum density, rho(p), from experimental data and its comparison with the theoretically calculated rho(p) and (iii) Analysis of the 2D ACPAR data in terms of Fermi surface parameters by applying Lock-Crisp-West theorem or the autocorrelation function. Usefulness of such comparison for obtaining information about the electron wave functions, Fermi surface and many-body enhancement effects is pointed out with the help of suitable examples. (Auth.)